Many of the industrially important ethylenically unsaturated monomers are highly susceptible to unwanted radical polymerization initiated either thermally or by adventitious impurities. Some examples of these monomers are acrylic and methacrylic acid, acrylate and methacrylate esters, acrylamide and methacrylamide, vinyl acetate and acrylonitrile. Premature polymerization may occur during manufacture, purification or storage of the monomer. Many of these monomers are purified by distillation. It is in this operation where premature polymerization is most likely to occur and to be the most troublesome. Methods to prevent or reduce the amount of such polymerization are thus highly desirable since the prevention or mitigation of such premature polymerization increases the yield of purified monomer and also insures against costly and potentially dangerous runaway polymerization in the plant.
Stable nitroxides are known in the art to be effective in preventing the premature radical polymerization of ethylenically unsaturated monomers. Some examples are seen in U.S. Pat. No. 3,747,988 in the stabilization of acrylonitrile. U.S. Pat. No. 3,488,338 describes the stabilization of chloroprene. British Patent No. 1,127,127 describes the stabilization of acrylic acid; and Japanese Sho 60-36501 describes the stabilization of acrylate and methacrylate esters. There is no mention of derivatives of 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl in these references.
German Application DE 195 10 184 A1 describes amide and formamide derivatives of 1-oxyl-2,2,6,6-tetramethyl-4-aminopiperidine as stabilizers for radically polymerizable monomers, but does not differentiate between aqueous and non-aqueous systems.
U.S. Pat. No. 5,545,786 describes the use of selected nitroxide compounds in the prevention of the premature polymerization of vinyl aromatic monomers such as styrene especially in the presence of oxygen. There is no disclosure or suggestion that such nitroxide compounds would be particularly effective in stabilizing acrylic monomers, such as acids, esters or amides, or vinyl acetate or acrylonitrile especially in the presence of water.
U.S. Pat. No. 5,254,760 discloses the use of selected nitroxide compounds in combination with an aromatic nitro compound for stabilizing vinyl aromatic monomers such as styrene. Again, there is no mention of aliphatic vinyl compounds or of the especial effectiveness of some selected nitroxide compounds in preventing the premature polymerization of such aliphatic vinyl monomers in the presence of water.
EP 697,386 generically discloses the use of selected nitroxyl compounds for preventing the premature polymerization of aromatic vinyl monomers such as styrene or aliphatic vinyl monomers such as acrylic monomers. Specifically, this reference teaches that 1-oxyl-4-acetylamino-2,2,6,6-tetramethylpiperidine alone or in combination with p-nitrosophenol or 2-methyl-4-nitrosophenol is effective in stabilizing styrene from premature polymerization. There is no mention that 1-oxyl-4-acetylamino-2,2,6,6-tetramethylpiperidine is used with an aliphatic vinyl monomer alone, and certainly no suggestion that said 1-oxyl-4-acetylamino-2,2,6,6-tetramethylpiperidine would be particularly effective with such aliphatic vinyl monomers in the presence of water.
EP 810,196 discloses the use inter alia of 1-oxyl-2,2,6,6-tetramethyl-4-acetylaminopiperidine in combination with a phosphine, such as triphenylphosphine, or a cobalt compound, such as cobalt acetate, as inhibitors to prevent the polymerization of (meth)acrylic acid or esters thereof. There is no teaching that 1-oxyl-2,2,6,6-tetramethyl-4-acetylaminopiperidine alone would be efficacious for that purpose.
Since, during the processes to produce and purify various ethylenically unsaturated monomers, water is often present during one of the process steps, there is a long felt need for the stable nitroxide inhibitor to be sufficiently water soluble or miscible to remain homogeneous in wet monomer streams and to prevent polymerization in the aqueous phase and yet for the inhibitor to be able to partition to such an extent that it can prevent polymerization in both the aqueous phase and in the organic monomer phase for inhibition protection throughout the entire process.